This class is an abstract implementation defining the basic API.
It refers to the elements as 'left', 'middle' and 'right'.

Subclass implementations may be mutable or immutable.
However, there is no restriction on the type of the stored objects that may be stored.
If mutable objects are stored in the triple, then the triple itself effectively becomes mutable.

toString

This uses Formattable to perform the formatting. Three variables may
be used to embed the left and right elements. Use %1$s for the left
element, %2$s for the middle and %3$s for the right element.
The default format used by toString() is (%1$s,%2$s,%3$s).